Circuit module

ABSTRACT

Circuit modules having one, two or three double-sided printed circuit boards (PCBs). Each PCB can have coils formed as spiral traces on both sides thereof. The coils can all be connected together, in series or in parallel, to function as a coil antenna. Alternatively, selected ones of the spiral traces can be connected to function as one winding of a transformer, the other coils functioning as another winding of the transformer and connected to an external antenna. Electronic components can be mounted to a lower one of the PCBs, and in embodiments having two or three PCBs, the PCB overlying the lower PCB has a central opening to accommodate the electronic components. A ferrite rod antenna may be disposed in the opening, aligned parallel to the PCBs, to provide a dual-directional antenna system.

TECHNICAL FIELD OF THE INVENTION

The invention relates to the interconnection and packaging of electroniccomponents and, more particularly, to a circuit module such as an RFtransponder for monitoring a condition within a pneumatic tire.

BACKGROUND OF THE INVENTION

As used herein, the term “circuit module” refers to an interconnectionsubstrate such as a printed circuit board (PCB) having electroniccomponents mounted thereto. A PCB is a multi-layer substrate, havingalternate layers of insulating material and conductive material. Theconductive material is patterned to have conductive “lines” or “traces”for routing signals (and power) from one location on the PCB to anotherlocation on the PCB. Electronic components are interconnected with oneanother by the conductive traces. Conductive traces may be disposed onboth sides (surfaces) of the PCB. Examples of electronic componentswhich may be incorporated in a circuit module include:

(a) “active” electronic components such as integrated circuit (IC)devices, and the like;

(b) “passive” electronic components such as resistors, capacitors, andinductors (including transformers), and the like;

(c) switches, relays and the like; and

(d) sensors, transducers and the like.

In some cases, a circuit module is intended to be disposed in a “harsh”environment, and it is desirable to isolate the electronic components ofthe circuit module from such an environment. An example of such anapplication for a circuit module is an RF transponder which is disposedwithin a pneumatic tire of a vehicle. In such applications, it isgenerally desirable to encapsulate or otherwise package the circuitmodule to isolate the electronic components from the environment.

As used herein, a “transponder” is an electronic apparatus (device)capable of monitoring a condition such as air pressure within apneumatic tire, and transmitting information (a signal) indicative ofthe monitored condition to an external device. The external device canbe either an RF (radio frequency) reader/interrogator or, simply an RFreceiver. A simple receiver can be used when the transponder is“active”, and has its own power source. A reader/interrogator wouldtypically be used when the transponder is “passive” and is powered by anRF signal from the reader/interrogator. In either case, in conjunctionwith the external device, the transponder forms a component of anoverall tire-condition monitoring/warning system.

In order to send and receive RF signals, a transponder must have anantenna. The antenna may either be incorporated into the transpondermodule itself, or it may be external to the transponder module andelectrically connected or coupled to it in a suitable manner.

U.S. Pat. No. 4,724,427 (Carroll; 1988), incorporated in its entirety byreference herein, discloses a transponder device. FIG. 9 of the patent,shows a topographical representation of a transponder chip (100) in anembodiment that includes an antenna coil (104) as part of a monolithicchip (102). As disclosed therein, the coil (104) is etched around theperiphery of the monolithic chip also known as the chip substrate (102).In the center of the coil (104) are found a custom logic circuit (106),a programmable memory array (108), and a memory control logic (110).Using the chip topography shown in this figure, a functionally completetransponder may be realized on a single semiconductor chip. (see column11, lines 7-22; numbers edited)

In a similar manner, U.S. Pat. No. 5,345,231 (09/1994) discloses acontactless inductive data-transmission system. FIG. 7 of this patentshows components of a chip having a substrate (52) which can bephotolithographically deposited along with antenna coils (50) which canbe in a plane above the semiconductor topography (51) of the chip.(column 7, lines 14-17) In both this patent and the aforementioned U.S.Pat. No. 4,724,427, the antenna coils are disposed around the peripheryof the IC chip, surrounding the components and periphery of the IC chip.

U.S. Pat. No. 5,574,470 (de Vall; 1996), incorporated in its entirety byreference herein, discloses a radio frequency identification transponderapparatus and method. A transponder is formed of a very thin flexibledielectric substrate (10) on opposite sides of which are formed first(26) and second (31) series-connected halves of a single antenna coilhaving ends connected to an integrated circuit die (50) mounted to a diebond site (46) at a corner of the substrate (10). Conductive vias(22,24) extend through the substrate (10) to effect connection betweenthe antenna halves (26,31), from one side of the substrate to the otherside of the substrate. Protective laminates (58,60) on either side ofthe substrate (10) are bonded to one another around the substrate edges,and also are bonded to one another at interior portions through a slot(20) formed in the substrate (10).

The following patents, all of which are incorporated in their entiretyby reference herein, are cited as being of interest: U.S. Pat. Nos.5,923,300; 5,894,006; 5,879,502; 5,870,066; 5,854,480; 5,461,545;5,420,757; 5,345,231; 5,313,365; 5,250,843; 5,223,851; 5,218,861;5,214,410; 5,181,975; 4,911,217; 4,851,855; 4,795,693; 4,724,427;4,628,148; 4,524,324; and 4,092,487.

SUMMARY OF THE INVENTION

According to the invention, a circuit module comprises a first printedcircuit board (PCB) having a first surface and a second surface,electronic components mounted to its first surface, a first longconductive spiral trace on its first surface and a second longconductive spiral trace on its second surface, wherein the second longconductive trace is connected in series with the first long conductivetrace to form a coil which is connected to the electronic components.

In a second embodiment, other PCBs are stacked parallel to the firstPCB, each having a first surface and a second surface, and each having along spiral conductive trace on its first surface and/or on its secondsurface. Still other PCBs having no traces may be stacked between thePCBs having traces in order to separate a long trace of one PCB from along trace of another PCB. A PCB adjacent to, but not possessing, theelectronic components may be in the form of a ring having an opening,wherein the opening forms a cavity for containing the electroniccomponents. The long conductive traces are attached all in series witheach other to form a single air core antenna coil, and the circuitmodule functions as a RF transponder.

In a third embodiment, the long conductive traces can be attached in twoseparate series to form two windings of a coupling transformer, in whichone winding (comprised of one or more traces connected in series) isconnected to the electronic components, and the other winding (alsocomprised of one or more traces connected in series) is connected to anexternal antenna. The external antenna can be a coil of wire disposedcircumferentially (360-degrees) around an inner surface of a pneumatictire, and the circuit module functions as a RF transponder.

In a variation of the second embodiment, a ferrite antenna comprising acoil of wire wound about a ferrite rod is disposed within the cavityformed by the opening in one of the PCBs. The ferrite antenna is alignedparallel with the PCBs, and thus perpendicular with respect to the axisof the coil formed by the long traces, to provide a dual-polarizedantenna system.

In a variation of any of the embodiments, the long spiral conductivetraces of some or all of the PCBs can be attached to each other inparallel, to improve performance by way of reduced electrical resistanceif the spiral coils are made of very fine pitched traces.

Any of the above embodiments of an RF transponder circuit module can beencapsulated within molding material disposed on all external surfacesof the circuit module. The molding material can be any rubber, ethylenepropylene diene monomer (EPDM) rubber, butyl rubber, natural rubber,neoprene and mixtures thereof, a mixture of chlorobutyl rubber andnatural rubber, or a mixture of styrene-butadiene rubber (SBR) andnatural rubber.

Other objects, features and advantages of the invention will becomeapparent in light of the following description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made in detail to preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. The drawings are intended to be illustrative, not limiting.Although the invention will be described in the context of thesepreferred embodiments, it should be understood that it is not intendedto limit the spirit and scope of the invention to these particularembodiments.

Certain elements in selected ones of the drawings may be illustratednot-to-scale, for illustrative clarity. The cross-sectional views, ifany, presented herein may be in the form of “slices”, or “near-sighted”cross-sectional views, omitting certain background lines which wouldotherwise be visible in a true cross-sectional view, for illustrativeclarity.

Elements of the figures are typically numbered as follows. The mostsignificant digits (hundreds) of the reference number usuallycorresponds to the figure number. Elements of FIG. 1 are typicallynumbered in the range of 100-199. Elements of FIG. 2 are typicallynumbered in the range of 200-299. Similar elements throughout thedrawings may be referred to by similar reference numerals. For example,the element 199 in a figure may be similar, and possibly identical tothe element 299 in an other figure. In some cases, similar (includingidentical) elements may be referred to with similar numbers in a singledrawing. For example, each of a plurality of elements 199 may bereferred to individually as 199 a, 199 b, 199 c, etc. Suchrelationships, if any, between similar elements in the same or differentfigures will become apparent throughout the specification, including, ifapplicable, in the claims and abstract.

The structure, operation, and advantages of the present preferredembodiment of the invention will become further apparent uponconsideration of the following description taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a cross-sectional view of a circuit module comprising one PCB,according to the invention;

FIG. 1A is a top view of the circuit module of FIG. 1, according to theinvention;

FIG. 2 is an exploded, cross-sectional view of a circuit modulecomprising two PCBs, according to the invention;

FIG. 2A is an exploded, cross-sectional view of a circuit modulecomprising two PCBs, according to the invention, connected to anantenna;

FIG. 3 is an exploded, cross-sectional view of a circuit modulecomprising three PCBs, according to the invention;

FIG. 3A is an exploded, cross-sectional view of another circuit modulecomprising three PCBs, according to the invention;

FIG. 4 is an exploded, cross-sectional view of another circuit modulecomprising three PCBs, according to the invention, connected to aferrite rod antenna;

FIG. 5A is an exploded, cross-sectional view of an injection moldingprocess for encapsulating a circuit module, according to the invention;

FIG. 5B is a cross-sectional view of a further step in the injectionmolding process for encapsulating a circuit module, according to theinvention; and

FIG. 5C is an exploded, cross-sectional view of a further step in theinjection molding process for encapsulating a circuit module, accordingto the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 (side cross-sectional view) and 1A (top view) illustrate acircuit module 100 comprising a printed circuit board (PCB) 102 havingconductive traces on both its top surface 102 a and its bottom surface102 b, and electronic components 104, 106 and 108 mounted to its topsurface 102 a and interconnected with one another.

A long conductive trace 110 is in the form of a spiral having a one end110 a and another end 110 b, and is disposed around a peripheral area ofthe top surface 102 a of the PCB 102. In the views of FIGS. 1 and 1A,TEN complete turns can be seen.

In a similar manner, a one long conductive trace 112 is in the form of aspiral having a one end 112 a and another end 112 b, and is disposed onthe bottom surface 102 b of the PCB 102. In the cross-sectional view ofFIG. 1, TWENTY turns can be seen.

The conductive traces 110 and 112 are connected in series with oneanother, and each of the conductive traces 110 and 112 forms a portionof an overall coil antenna for the circuit module 100. For example, thecomponent 104 is connected to the end 110 a of the trace 110. The end110 b of the trace 110 is connected by a via 114 through the PCB 102 tothe end 112 a of the trace 112. The end 112 b of the trace 112 isconnected by a via 116 through the PCB 102 to the component 106.

The conductive traces 110 and 112, and those discussed hereinbelow, arein the form of loops having many turns. These loops can be characterizedas a planar coil which lays flat on the surface of the respective PCB ina form such as a spiral. As is known for coil antennas, the direction ofcurrent flow around the coil determines the direction of the radiatedmagnetic field, and vice-versa for a receiving antenna. Therefore, forthe example hereinabove wherein traces 110 and 112 are connected inseries to form at least part of an overall coil antenna, the spirals forthe two traces 110 and 112 must be laid out so that the current flows inthe same direction in the trace 110 and in the trace 112 portion of theantenna. For example, if the trace 110 spirals from end 110 a to end 110b in a clockwise direction (looking down at the top PCB surface 102 a),then the trace 112 must also spiral in the same clockwise direction fromend 112 a to end 112 b.

It is within the scope of this invention that an overall coil antennamay be formed by connecting coils such as trace 110 and trace 112 inparallel with each other. This may be desirable in cases where, forexample, very fine-pitched traces 110 and 112 are used, producing highelectrical resistance in each coil. For a parallel connection, ends 110a and 112 b would be connected together and to one antenna endconnection such as component 106, and ends 110 b and 112 a would beconnected together and to the other antenna end connection such ascomponent 104. Such a parallel connection is not illustrated in FIG. 1,but can be seen in the parallel-connected traces 350 and 352 of FIG. 3.Known techniques could be employed to connect component 104 to trace end110 b (e.g., extending an insulated wire between the two, preferablycrossing the turns of trace 110 at right angles, as illustrated for wire227 of FIG. 2 which crosses the turns of trace 230). Now, in order tomaintain proper current direction in a parallel-connected antenna formedfrom traces 110 and 112, if the trace 110 spirals from end 110 a to end110 b in a clockwise direction (looking down at the top PCB surface 102a), then the trace 112 must also spiral in the same clockwise directionfrom end 112 b to end 112 a.

For a transmitting loop, the driving point voltage and current isproportional to radiation resistance (Rr) of the loop and is given bythe following equation (eqn 1):

Rr=[6/(Mo/Eo)]² (2L)⁴ (NA)²

where:

(Mo/Eo)⁻² is the wave impedance of the space

2/L is the propagation constant in the space

N is the number of turns in the loop

A is the area of the loop

For a receiving loop, the voltage (Vr) developed at its open-circuitedterminals is given by the following equation (eqn 2):

Vr=jwNAB_(z)

where:

j is the square root of minus 1;

w is angular frequency (2 f);

N is the number of turns in the loop;

A is the area of the loop; and

Bz is the component of incident magnetic density normal to the plane ofthe loop.

As can be seen by the expressions above, whether the conductive traces110, 112 serve as a transmitting loop or a receiving loop, the RFtransmission and receiving strength is directly related to the number ofturns and the area circumscribed by the turns.

FIG. 2 illustrates a circuit module 200 comprising two printed circuitboards (PCBs) 202 and 222. The PCB 202 (compare 102) has conductivetraces on both its top surface 202 a and its bottom surface 202 b, andelectronic components 204, 206 and 208 mounted to its top surface 202 aand interconnected with one another.

A one long conductive trace 210 (compare 110) is in the form of a spiralhaving a one end 210 a and another end 210 b, and is disposed around aperipheral area of the top surface 202 a of the PCB 202. In thecross-sectional view of FIG. 2, TEN turns can be seen.

In a similar manner, a one long conductive trace 212 (compare 112) is inthe form of a spiral having a one end 212 a and another end 212 b, andis disposed on the bottom surface 202 b of the PCB 202. In thecross-sectional view of FIG. 2, TWENTY turns can be seen.

The PCB 222 is in the form of a ring, having a central opening 224, andhas a top surface 222 a and a bottom surface 222 b. A one longconductive trace 230 (compare 210) is in the form of a spiral having aone end 230 a and another end 230 b, and is disposed around a peripheralarea of the top surface 222 a of the PCB 222. In the cross-sectionalview of FIG. 2, TEN turns can be seen.

The conductive traces 210, 212 and 230 may be connected in parallel (notshown) or in series with one another (as illustrated) so that each formsa portion of a single overall coil antenna for the circuit module 200.For example, the component 204 is connected to the end 210 a of thetrace 210. The end 210 b of the trace 210 is connected by a via 214through the PCB 202 to the end 212 a of the trace 212. The end 212 b ofthe trace 212 is connected by a via 216 through the PCB 202 to a surfacetrace 217 which contacts a via 226 through the PCB 222 to the end 230 aof the trace 230. The end 230 b of the trace 230 is connected by a wire227 to via 228 through the PCB 222 to connect with a surface trace 207and thereby to the component 206. For clarity of illustration, the trace207 is shown as if it were raised above trace 217, whereas in realitythe traces 207 and 217 would usually be of equivalent height andseparated horizontally, such as trace 207 being behind trace 217 in theview of FIG. 2. The wire 227 is suitably insulated from the spiral turnsof trace 230 and preferably crosses the turns at right angles. Suitableinsulation includes, for example, an air gap, enamel or PVC coating onthe wire 227, an/or an insulating coating over the PCB 222 surface 222 aand over the trace 230.

One having ordinary skill in the art to which the invention most nearlypertains will understand how, in this and other embodiments of theinvention presented herein, connections are made between PCBs (from onePCB to another), using pins, connectors and the like. Generally, in thisand other embodiments of the invention presented herein, when PCBs arestacked one atop the other, they preferably have approximately the sameoutside dimensions. In other words, they are preferably the same sizeand shape as one another.

Alternatively to the embodiment illustrated in FIG. 2, FIG. 2Aillustrates the circuit module 200′, having conductive traces 210′ and212′ (compare 110 and 112) on the lower PCB 202′ (compare 102) which maybe series-connected with one another and connected to the electroniccomponents 204′ and 206′, respectively, in the manner described for thecircuit module 100, to form a winding of an air-gap transformer(“air-gap” is commonly understood to include non-conductive,non-magnetic materials such as the PCB material filling the transformergap), with the conductive trace 230′ on the upper PCB 222′ not beingconnected to the traces 210′ and 212′ but rather serving as the otherwinding of the air-gap transformer. Such a transformer can serve as acoupling transformer which is connected to an antenna 260 for thecircuit module. The ends 230 a′ and 230 b′ of the conductive trace 230′are shown connected to the antenna 260. The circuit module 200′ issuitably an RF transponder, and the antenna 260 is suitably a coil ofwire disposed circumferentially (360-degrees) around an inner surface ofa pneumatic tire (not shown).

The lower PCB 202′ has an upper surface 202 a′ and a lower surface 202b′. The upper PCB 222′ has an upper surface 222 a′ and a lower surface222 b′, and a central opening 224′. The trace 210′ has two ends 210 a′and 210 b′, and the trace 212′ has two ends 212 a′ and 212 b′.

FIG. 3 illustrates a circuit module 300 comprising three circuit boards(PCBs) 302, 322 and 342.

The lower PCB 302 (compare 102) has conductive traces on both its topsurface 302 a and its bottom surface 302 b, and electronic components304, 306 and 308 mounted to its top surface 302 a and interconnectedwith one another.

A one long conductive trace 310 (compare 110) is in the form of a spiralhaving a one end 310 a and another end 310 b, and is disposed around aperipheral area of the top surface 302 a of the PCB 302. In thecross-sectional view of FIG. 3, TEN turns can be seen.

In a similar manner, a one long conductive trace 312 (compare 112) is inthe form of a spiral having a one end 312 a and another end 312 b, andis disposed on the bottom surface 302 b of the PCB 302. In thecross-sectional view of FIG. 3, TWENTY turns can be seen.

In this example, the conductive traces 310 and 312 are connected inseries with one another, and each form a portion of a transformerwinding for the circuit module 300. For example, the component 304 isconnected to the end 310 a of the trace 310. The end 310 b of the trace310 is connected by a via 314 through the PCB 302 to the end 312 a ofthe trace 312. The end 312 b of the trace 312 is connected by a via 316through the PCB 302 to the component 306.

The middle PCB 322 is in the form of a ring, having a central opening324, and has a top surface 322 a and a bottom surface 322 b. In thisexample, the PCB functions primarily as a spacer, having no electricalfunctionality associated therewith. (Compare the ring-like PCB 222 whichhas conductive traces on a surface thereof).

The top PCB 342 has an upper surface 342 a and a lower surface 342 b,and is disposed atop the PCB 322 so as to cover the opening 324 in themiddle of the PCB 322.

A one long conductive trace 350 (compare 112, 110) is in the form of aspiral having a one end 350 a and another end 350 b, and is disposed onthe top surface 342 b of the PCB 342. In the cross-sectional view ofFIG. 3, TWENTY turns can be seen. In a similar manner, a one longconductive trace 352 (compare 112) is in the form of a spiral having aone end 352 a and another end 352 b, and is disposed on the bottomsurface 342 b of the PCB 342. In the cross-sectional view of FIG. 3,TWENTY turns can be seen.

In this example, the conductive traces 350 and 352 are connected inparallel with one another, and each form a portion of a transformerwinding for the circuit module 300. For example, an antenna 360 (compare260) is connected to the end 350 a of the trace 350 and also to the end352 b of the trace 352 by way of a via 356 (compare 316) through the PCB342. The other end of the antenna 360 is connected to the end 350 b ofthe trace 350 and also to the end 352 a of the trace 352 by way of a via354 (compare 314) through the PCB 342. In this manner, an air-gaptransformer is formed, one winding comprising the series-connectedtraces 310 and 312, the other winding comprising the parallel-connectedtraces 350 and 352. The circuit module 300 is suitably an RFtransponder, and the antenna 360 is suitably a coil of wire disposedcircumferentially (360-degrees) around an inner surface of a pneumatictire (not shown).

FIG. 3A illustrates a circuit module 300′ comprising three printedcircuit boards (PCBs) 302′, 322′ and 342′.

The lower PCB 302′ (compare 202) has conductive traces on both its topsurface 302 a′ and its bottom surface 302 b′, and electronic components304′, 306′ and 308′ mounted to its top surface 302 a′ and interconnectedwith one another.

A one long conductive trace 310′ (compare 210) is in the form of aspiral having a one end 310 a′ and another end 310 b′, and is disposedaround a peripheral area of the top surface 302 a′ of the PCB 302′. Inthe cross-sectional view of FIG. 3A, TEN turns can be seen.

In a similar manner, a one long conductive trace 312′ (compare 212) isin the form of a spiral having a one end 312 a′ and another end 312 b′,and is disposed on the bottom surface 302 b′ of the PCB 302′. In thecross-sectional view of FIG. 3A, TWENTY turns can be seen.

The middle PCB 322′ (compare 222) is in the form of a ring, having acentral opening 324′, and has a top surface 322 a′ and a bottom surface322 b′. A one long conductive trace 330 (compare 230) is in the form ofa spiral having a one end 330 a and another end 330 b, and is disposedaround a peripheral area of the top surface 322 a′ of the PCB 322′. Inthe cross-sectional view of FIG. 3A, TEN turns can be seen. The end 330a is connected to a contact pad 331, and the end 330 b is connected to avia 326. A second via 328 passes through the PCB 322′ somewhere near theedge of the central opening 324′ and does not make electrical contactwith any part of the trace 330.

The top PCB 342′ has an upper surface 342 a′ and a lower surface 342 b′,and is disposed atop the PCB 322′ so as to cover the opening 324′ in themiddle PCB 322′.

A one long conductive trace 350′ (compare 350) is in the form of aspiral having a one end 350 a′ and another end 350 b′, and is disposedon the top surface 342 a′ of the PCB 342′. In the cross-sectional viewof FIG. 3A, TWENTY turns can be seen.

The conductive traces 310′, 312′, 330, and 350′ may be connected inparallel (not shown) or in series with one another (as illustrated) sothat each forms a portion of a single overall coil antenna for thecircuit module 300′. For example, the component 304′ is connected to theend 310 a′ of the trace 310′. The end 310 b′ of the trace 310 isconnected by a via 314′ through the PCB 302′ to the end 312 a′ of thetrace 312′. The end 312 b′ of the trace 312′ is connected by a via 316′(compare 216) through the PCB 302′ to a surface trace 317 which contactsthe via 326 (compare 226) through the PCB 322′ to the end 330 b of thetrace 330. The end 330 a of the trace 330 is connected to a contact 331which connects to a via 354′ through the PCB 342′ to the end 350 a′ ofthe trace 350′. The end 350 b′ of the trace 350′ is connected by a via356′ through the PCB 342′ to a surface trace 357 with an end 357 a whichis placed to connect with via 328 through the PCB 322′ to connect with asurface trace 307 and thereby to the component 306. For clarity ofillustration, the trace 307 is shown as if it were raised above trace317, whereas in reality the traces 307 and 317 would usually be ofequivalent height and separated horizontally, such as trace 307 beingbehind trace 317 in the view of FIG. 3A.

It is within the scope of this invention that any or all of the circuitboards in the module 300′ have long (elongate) conductive traces on oneor both of their surfaces, and that these elongate conductive traces maybe in the form of spirals, including square spirals, and that theconductive traces may be connected with one another in any manner, suchas in series or in parallel with one another, or such as somefunctioning as a one winding of a coupling transformer and the othersacting as the other winding of a coupling transformer, however aparticular application for the circuit module may dictate.

The circuit modules 300 and 300′ of FIGS. 3 and 3A, respectively, eachhave three PCBs stacked one atop the other. In both cases, the middlePCB 322, 322′ is in the form of a ring, having a central opening 324,324′, respectively. As is evident from the illustrations, the top PCB342, 342′ covers the opening 324, 324′, respectively. The opening 324,324′ thus forms a cavity wherein the components 304, 306, 308 (and 304′,306′, 308′) are contained within the assembly of three PCBs. With theexception that it is not covered, the opening 224, 224′ in the PCB 222,222′ of the circuit module 200, 200′ shown in FIGS. 2 and 2A also formsa cavity for containing the electronic components 204, 206, 208, 204′,206′, 208′ mounted to the lower PCB 202, 202′. These openings 224, 224′,324, 324′ are suitably disposed in the center of the respective PCBs222, 222′, 322, 322′, but it is within the scope of the invention thatthe openings 224, 224′, 324, 324′ are disposed other than at the centerof the respective PCBs 222, 222′, 322, 322′.

Coils, whether they be antenna coils or transformer windings, formed bythe elongate traces (110, 112, 210, 210′, 212, 212′, 230, 230′, 310,310′, 312, 312′, 330, 350, 350′) on the PCBs (102, 202, 202′, 222, 222′,302, 302′, 322, 322′, 342, 342′) have an axis which is normal to thesurface of the PCB on which they are disposed. As viewed in the figures,the axis would be vertical on the sheet. Also, as described hereinabove,the spiraling direction of each of these coils formed by the elongatetraces 110, 112, 210, 210′, 212, 212′, 230, 230′, 310, 310′, 312, 312′,330, 350, 350′ is suitably determined in order to maintain parallel,same-direction current in adjacent connected coils, taking into accountwhether the coils are connected in series or in parallel.

With particular regard to elongate traces forming an antenna for therespective circuit module, it is within the scope of the invention thatan additional antenna or transformer element may be included in thecircuit module.

FIG. 4 illustrates a circuit module 400 which is substantially identicalto the previously-described circuit module 300′ of FIG. 3A. Wherefeatures of FIG. 4 are identical to corresponding features in FIG. 3A,the reference numbers for those features will be the same in the twofigures. Features which have been added in FIG. 4 have reference signsin the 400 series, and modified features have a reference number whichis the same as in FIG. 3A except for a double prime.

FIG. 4 illustrates an alternative embodiment of the coil placement,wherein the coil formed from trace 330″ (compare 330) is placed on thebottom surface 342 b′ of the top PCB 342′ so that the middle PCB 322′has no traces on it and serves mainly as an insulator/spacer between thebottom PCB 302′ and the top PCB 342′. The vias 326 and 328 remain in themiddle PCB 322′ in order to carry current between the bottom PCB 302′and the top PCB 342′. The via 326 is now only connected to the end 330b″ of the trace 330″ by way of contact pad 431 when the PCBs areassembled together, and the end 330 a″ of the trace 330″ is now fixedlyconnected to the via 354′, thereby eliminating the contact pad 331.

A ferrite rod antenna 410 comprising a coil of wire 412 wound about agenerally-cylindrical ferrite rod 414 is disposed within the cavityformed by the opening 324′. The rod 414 is oriented so that its axis 420(hence the axis of the coil of wire 412) is parallel to the surface 302a′ of the PCB 302′. The coil of wire 412 has two ends 416 and 418 whichare connected to respective terminals 426 and 428 on the PCB 302′. Theferrite rod antenna 410 may be used in lieu of the antenna coils formedby the elongate traces 310′, 312′, 330″, 350′ on the PCBs 302′, 322′,342′, but preferably is used in conjunction with the antenna coilsformed by the elongate traces 310′, 312′, 330″, 350′.

It is within the scope of the invention that the wire 412 is replaced byconductive traces on the surfaces 302 a′ and 342 b′ of the lower andupper PCBs 302′ and 342′, respectively, and, optionally, conductivetraces on the sidewall(s) of the cavity formed by the opening 324′ inthe middle PCB 322′. This can be structured with or without the ferriterod.

As mentioned above, the coils formed by the elongate traces 310′, 312′,330″, 350′ on the PCBs 302′, 322′, 342′ have an axis which is normal tothe surfaces of the PCBs on which they are disposed. Therefore, the axis420 of the ferrite rod antenna 410 is orthogonal to the axis of theantenna coils formed by spiral elongate traces 310′, 312′, 330″, 350′ onthe PCBs. In this manner, using the ferrite rod antenna 410 inconjunction with the antenna coil(s) formed by the elongate traces 310′,312′, 330″, 350′, an omni-directional (or, dual-polarized ordirectional) antenna system is provided for the transponder.

Various benefits will accrue when using such an omni-directional ordual-directional antenna system in a circuit module (400) serving as anRF transponder disposed within a pneumatic tire. Preferably, thetransponder (circuit module 400) would be mounted within the tire sothat the axis 420 of the ferrite rod antenna 410 extends axially (frombead-to-bead) across the tread of the tire, i.e., is parallel to theaxis of the tire.

Commonly-owned U.S. Pat. No. 5,181,975 and U.S. Pat. No. 5,218,861,incorporated in their entirety by reference herein, disclose improvingcoupling to a tire-mounted transponder by utilizing an annular bead ofthe tire as the primary winding of a transformer. A transponder isdisposed near the annular bead, and has a coil antenna that is looselyelectromagnetically coupled to the annular bead, and acts as thesecondary winding of the transformer.

Despite the “360-degree” readability of a transponder which can resultfrom using an annular bead or the like in the tire as a couplingelement, it is suspected that coupling from a transponder in a rotatingtire, via an annular bead or the like, to an externally fixed antennacan result in non-uniform coupling, including phase shifts. It is, ofcourse, generally desirable to ensure uniform, or at least adequate,coupling of RF signals between an on-board interrogator and atire-mounted transponder, irrespective of the orientation of the wheelvis-a-vis the vehicle-mounted antenna. By using the omni-directionalantenna system of the present invention in a circuit module (400)serving as an RF transponder disposed within a pneumatic tire, asdescribed hereinabove, coupling can be enhanced and rendered moreuniform.

ENCAPSULATING THE CIRCUIT MODULE

U.S. Pat. No. 5,420,757 (Eberhardt, et al.; 1995), incorporated in itsentirety by reference herein, discloses a method of producing a RFtransponder with a molded environmentally sealed package. The componentsare mounted on a lead-type substrate frame, and are encapsulated in aplastic housing using a conventional plastic molding process utilizing asuitable plastic or epoxy (column 6, lines 56-62). The leads whichsupport the transponder in the mold during the molding process areeventually severed to provide a leadless package. As disclosed in thepatent, the frame may be a PCB formed to have supporting arms which aresevered at the periphery of the respective plastic molded housings.

Injection molding is a well-known process. Typically, a two-part mold ofan injection molding machine has a cavity which is in the shape of thedesired final product. Molten or fluid material is injected into thecavity via gates to fill the cavity, and encapsulate any object whichwas previously inserted into the cavity.

FIGS. 5A, 5B and 5C illustrate an injection molding process forencapsulating any of the circuit modules described hereinabove—forexample, the circuit module 300.

FIG. 5A shows a mold 500 while open. The mold 500 comprises two moldhalves, an upper mold half 502 and a lower mold half 504. An innersurface 502 a of the upper mold half 502 has a recess 506. An innersurface 504 a of the lower mold half 504 has a recess 508. When the moldhalves 502 and 504 are brought together, as indicated by the arrows inFIG. 5A, the recesses 506 and 508 form a cavity. The circuit module 300is disposed in any suitable manner, such as supported by pins or posts510, within the cavity.

FIG. 5A shows the mold 500 when closed. The circuit module 300 issmaller than the cavity, there being spaces between the circuit moduleand the walls of the cavity.

The lower mold half 504 is provided with a gate 512 for injectingmolding material into the mold cavity, as indicated by the arrow 514.The upper mold half 502 is provided with a vent hole 516 for venting gas(e.g., air) from the mold cavity, as indicated by the arrow 518. This isbest viewed in FIG. 5B which shows the mold cavity partially(approximately half, e.g., the left half as viewed in the figure) filledwith molding material 520, and the circuit module 300 partiallyencapsulated. Once the cavity is filled, the molding material is moldedto shape and allowed to solidify, by cooling or curing to a desireddegree prior to removal from the mold. Heat can be applied to the moldif necessary (depending on the material used).

FIG. 5C shows the mold 500 opened up upon completion of the moldingprocess. At the completion of the molding process, the circuit module300 is fully and completely encapsulated with molding material 520, andthe mold halves 502 and 504 are separated, as indicated by the arrows inFIG. 5C, resulting in the encapsulated circuit module 530 illustrated inFIG. 5C which has molding material on all of its external (exposed)surfaces.

Suitable molding materials for encapsulating the circuit module includerubber selected from the group consisting of essentially ethylenepropylene diene monomer (EPDM) rubber, butyl rubber, natural rubber,synthetic rubbers, neoprene and mixtures thereof, such as, for example,a mixture of halobutyl rubber and natural rubber, poul-butadiene rubberand natural rubber or a mixture of styrene-butadiene rubber (SBR) andnatural rubber. These rubber compositions can typically be cured byheating to a desired temperature in the order of 150 degrees (C) and fora period of time depending upon the curing system used, accelerators,and the degree of curing desired prior to removal of the product fromthe mold.

While the invention has been described in combination with embodimentsthereof, it is evident that many alternatives, modifications, andvariations will be apparent to those skilled in the art in light of theforegoing description. Accordingly, it is intended to embrace all suchalternatives, modifications and variations as fall within the spirit andscope of the appended claims.

What is claimed is:
 1. A circuit module comprising: a first printedcircuit board having electronic components on a first surface thereof,having a first conductive trace on the first surface thereof, and havinga second conductive trace on a second surface thereof, wherein thesecond conductive trace is connected to the first conductive trace; andto selected ones of the electronic components; a second printed circuitboard in the form of a ring, disposed on the first surface of the firstprinted circuit board and having an opening for containing theelectronic components; an antenna; a third printed circuit board,disposed atop the second printed circuit board and having a thirdconductive trace disposed on a surface of the third printed circuitboard; wherein: the third conductive trace is connected to the antenna;and the first and second conductive traces constitute a first winding ofan air-gap transformer; and the third conductive trace constitutes asecond winding of the air-gap transformer.
 2. Circuit module, accordingto claim 1, further comprising: a fourth conductive trace on an oppositesurface of the third printed circuit board, wherein the fourthconductive trace is connected to the third conductive trace.
 3. Circuitmodule, according to claim 2, wherein: the third conductive trace is inthe form of a spiral, the fourth conductive trace is in the form of aspiral.
 4. Circuit module, according to claim 2, wherein: the third andfourth conductive traces are connected to the first and secondconductive traces by vias extending through the second printed circuitboard.
 5. A circuit module comprising: a first printed circuit boardhaving electronic components on a first surface thereof, having a firstconductive trace on the first surface thereof, and having a secondconductive trace on a second surface thereof, wherein the secondconductive trace is connected to the first conductive trace; and toselected ones of the electronic components; a second printed circuitboard in the form of a ring, disposed on the first surface of the firstprinted circuit board and having an opening for containing theelectronic components; an antenna; a third printed circuit board,disposed atop the second printed circuit board and having a thirdconductive trace disposed on a surface of the third printed circuitboard; wherein: the third conductive trace is connected to the antenna;and the first and second conductive traces constitute a first winding ofan air-gap transformer; the third conductive trace constitutes a secondwinding of the air-gap transformer; further comprising: a fourthconductive trace on an opposite surface of the third printed circuitboard, wherein the fourth conductive trace is connected to the thirdconductive trace; a ferrite antenna comprising a coil of wire woundabout a ferrite rod disposed within the opening in the second printedcircuit board.
 6. A circuit module comprising: a first printed circuitboard having electronic components on a first surface thereof, having afirst conductive trace on the first surface thereof, and having a secondconductive trace on a second surface thereof, wherein the secondconductive trace is connected to the first conductive trace; and toselected ones of the electronic components; a second printed circuitboard in the form of a ring, disposed on the first surface of the firstprinted circuit board and having an opening for containing theelectronic components; and a ferrite antenna comprising a coil of wirewound about a ferrite rod disposed within the opening in the secondprinted circuit board.
 7. Circuit module, according to claim 6, wherein:the first and second conductive traces have an axis which is normal tothe surface of the first printed circuit board the ferrite rod has anaxis which is parallel to the surface of the first printed circuitboard.
 8. Circuit module, according to claim 6, wherein: the ferrite rodhas an axis which is orthogonal to an axis of the first and secondconductive traces.